The present invention relates to a method for forming an aspherical optical element for use in various measuring equipments.
In order to form an aspherical optical element at reduced cost, there has heretofore been used a resin replica method. In accordance with this method, the surface of an aspherical optical element which has been polished with a high precision as a master mold is treated with a release agent. The reversed shape of the master mold is then transferred to a resin provided on a spherical or flat substrate which is a negative matrix to form a negative mold. Thereafter, the reversed shape of the negative mold is transferred to a resin provided on a spherical or flat substrate to form an aspherical optical element having the same shape as that of the master mold.
Another resin replica method includes forming a negative mold the reversed shape of which is the same as that of the desired aspherical optical element, and then transferring the reversed shape of the negative mold to a resin provided on a spherical or flat substrate to form an aspherical optical element.
There have been proposed a method which includes forming the substrate of the product itself into a rough aspherical form to give a high precision (see, e.g., Patent Reference 1) and a method which includes forming a negative mold the shape of which is the same as the reversed shape of master mold (see, e.g., Patent Reference 2).    [Patent Reference 1]
JP-A-No. Sho63-157103    [Patent Reference 2]
Japanese Patent Application No. 2003-362489
In the case where an aspherical optical element is formed by a resin replica method using a spherical or flat substrate having the shape close to the aspherical shape of an optical element which is a master mold, when the difference in shape between the mold and the substrate is large, the resulting cure shrinkage of resin makes it difficult to provide the aspherical optical element thus formed with a high precision. The estimated value of deviation of precision is the product of the difference in shape and the cure shrinkage. For example, when the difference in shape is 30 μm and a resin having a cure shrinkage ratio of 2% is used, the precision is deteriorated by 0.6 μm (=30×2/100) merely by cure shrinkage. Since the reversed shape of the negative mold the precision of which has thus been deteriorated is used in another replica method, the surface precision of the product is further deteriorated.
In the case where the substrate of the product itself is formed into a rough spherical form as disclosed in the above cited Patent Reference 1 to solve the aforementioned problems, it is unavoidable that the production process is complicated as compared with that for flat or spherical substrate although it is a rough aspherical substrate. It is not wise from the standpoint of cost to use an aspherical optical element for all products.
There may be also used a method involving the use of a negative mold substrate the shape of which is the same as the reversed shape of master mold as disclosed in the above cited Patent Reference 2. However, in even this method, the problem of deterioration of surface precision due to cure shrinkage of resin is left unsolved.